Induction heat treatment of workpieces

ABSTRACT

An apparatus and process are provided for induction heating of a workpiece. The workpiece is moved through an inductor to inductively heat treat the workpiece with electric power of varying frequency and duty cycle or amplitude control to control the magnitude of electric power as the frequency changes. Alternatively the workpiece may be stationary and the inductor can be moved along the workpiece, or combined and coordinated movement of both the workpiece and inductor can be used.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional application of pending U.S. application Ser. No.11/760,772, filed Jun. 10, 2007, which is a continuation-in-part of U.S.application Ser. No. 11/261,097, filed Oct. 28, 2005, which claimspriority to provisional patent application no. 60/623,413, filed Oct.30, 2004, the entirety of each of which applications are incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention relates to induction heat treatment of continuousor discrete workpieces wherein pulse width modulation control oramplitude control is used to control induction heat treatment of theworkpieces.

BACKGROUND OF THE INVENTION

Elongated workpieces, such as a drive shaft, require heat treatment ofselected features on the workpiece. For example, a first feature, suchas a pinion gear, may be provided at one end of a drive shaft, and asecond feature, such as a universal coupling may be provided at theother end. The gear and coupling are of different physicalconfigurations and require different heat treatment patterns formetallurgical hardening of these components. Additionally a heat-treatedfeature may need to be tempered after heat treating to relievemetallurgical stresses in the material of the feature.

One method of heat treating the workpiece and features on the workpieceis by electric induction scanning (or progressive) heat treatment. Inthis process, the workpiece generally moves through one or more scaninductors, although in other arrangements, the workpiece may bestationery and the one or more scan inductors (coils) may move along thelength of the workpiece. AC power is applied to the scan inductor tocreate a magnetic field around the inductor. The field magneticallycouples with the workpiece to inductively heat the workpiece. AC powerto the scan inductor may be varied as the workpiece passes through theinductor. For example U.S. Pat. No. 3,743,808 teaches controlling theinduction power and/or the scanning velocity of the scan inductor bycomparing instantaneous power and the instantaneous velocity with aknown energy distribution profile. The rate at which the workpiece movesthrough the inductor (scan rate) can be used to control the degree ofheating at the cross section of the workpiece that is coupled with themagnetic field.

Induction heat depth of penetration (induced current depth ofpenetration, δ) of a workpiece can be calculated from the formula:

$\delta = {503\sqrt{\frac{\rho}{\mu\; F}}}$

where δ is in meters; ρ is the electrical resistivity of the workpiecein ohm-meters; μ is the relative magnetic permeability of the workpiece;and F is the frequency of the supplied induction power in Hertz.Therefore depth of penetration is inversely proportional to the squareroot of the frequency of the applied current. If the workpiece has twofeatures with a first feature that requires heating to a shallow depthof penetration (e.g. 2.5 mm), and a second feature that requires heatingto a deeper depth of penetration (e.g. 4.5 mm), the conventional methoduses an inverter with a fixed output frequency, for example 10,000Hertz, to achieve the shallower depth of penetration. From the aboveequation, the inverter's output frequency should be lower than 10,000 Hzfor the deeper depth of penetration of the second feature of theworkpiece, but since the frequency is fixed, the induction heat scan ofthe second feature must be slowed down to allow for deeper heatpenetration by heat conduction into the second feature. Further becauseof the slower scan rate, inverter output power to the induction coilmust be reduced to avoid overheating of the surface of the secondfeature. Also a heat-treated feature may require tempering of theheat-treated feature to reduce stresses in the feature. Typically thefeature is first heat treated in a first scan with low power and fixedhigh frequency to heat treat to the required depth of penetration, andthen heated in a second scan with fixed low frequency to temper thefeature.

One object of the present invention is to vary the output frequency ofthe inverter while adjusting the output power level of the inverter bypulse width modulation, as required to inductively heat treat and/ortemper various features of a workpiece to different depths ofpenetration in an induction scan of the workpiece.

Another object of the present invention is to control the outputfrequency of the power source to achieve optimal induction heating bycontrol of the depth of penetration.

Another object of the present invention is to vary the output frequencyof the inverter while adjusting the output power level of the inverterby pulse width modulation or amplitude control, as required toinductively heat treat and/or temper a workpiece to varying degrees.

BRIEF SUMMARY OF THE INVENTION

In one aspect the present invention is a method of induction heating oneor more multiple features of a workpiece. Electric power is supplied toan inductor to generate an ac magnetic field around the inductor. Eachof one or more multiple features of the workpiece are sequentiallypositioned in the vicinity of the magnetic field to magnetically couplethe positioned feature to the magnetic field whereby the positionedfeature is inductively heat treated and the impedance of the suppliedelectric power is matched with the impedance of the workpiece. Thefrequency of the electric power is selectively varied while each one ofthe one or more multiple features of the workpiece are sequentiallypositioned in the vicinity of the magnetic field and the magnitude ofthe electric power is selectively varied by changing the duty cycle ofthe electric power while each one of the multiple features of theworkpiece are sequentially positioned in the vicinity of the magneticfield and the frequency of the electric power is adjusted.

In another aspect the present invention is an apparatus for inductionheat treatment of a continuous or discrete workpiece. A power source hasan ac output with pulse width modulation control and an inductor isconnected to the ac output to generate an ac magnetic field. Means areprovided for moving the continuous or discrete workpiece through theinductor to magnetically couple progressive cross sections of thecontinuous or discrete workpiece with the magnetic field. Means are alsoprovided for selectively adjusting the frequency of the ac output wheneach one of the progressive cross sections is coupled with the magneticfield for induction heat treatment responsive to a parametric change ineach one of the progressive cross sections, and for selectivelyadjusting the power of the ac output by changing the duty cycle of theac output when each one of the progressive cross sections is coupledwith the magnetic field for induction heat treatment and the frequencyof the ac output is adjusted responsive to a parametric change in eachone of the progressive cross sections.

In another aspect the present invention is a method of induction heattreatment of a continuous or discrete workpiece. Electric power issupplied to at least one inductor to generate an ac magnetic fieldaround the at least one inductor. The workpiece is moved through theinductor to magnetically couple with progressive cross sections of theworkpiece and subject the progressive cross sections to a heattreatment. The frequency of the electric power is selectively variedwhile each one of the progressive cross sections are sequentiallypositioned in the vicinity of the magnetic field, and the magnitude ofthe electric power is selectively varied by changing the duty cycle ofthe electric power while each one of the progressive cross sections ofthe workpiece are sequentially positioned in the vicinity of themagnetic field and the frequency of the electric power is adjusted.

In another aspect the present invention is an apparatus for inductionheat treatment of a continuous or discrete workpiece. An inverter has adc input and an ac output with an inductor connected to the ac output togenerate an ac magnetic field. Means are provided for moving thecontinuous or discrete workpiece through the inductor to magneticallycouple progressive cross sections of the continuous or discreteworkpiece with the magnetic field. Means are also provided forselectively adjusting the frequency of the ac output when each one ofthe progressive cross sections is coupled with the magnetic field forinduction heat treatment responsive to a parametric change in each oneof the progressive cross sections, and means are also provided forselectively adjusting the power of the ac output by changing theamplitude to the dc input of the inverter when each one of theprogressive cross sections is coupled with the magnetic field forinduction heat treatment and the frequency of the ac output is adjustedresponsive to a parametric change in each one of the progressive crosssections.

In another aspect, the present invention is an apparatus for, and methodof, supplying ac power with varying frequency and duty cycle oramplitude control to an induction coil based upon the heatingrequirements of the cross section of a workpiece moving through thecoil.

Other aspects of the invention are set forth in this specification andthe appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing brief summary, as well as the following detaileddescription of the invention, is better understood when read inconjunction with the appended drawings. For the purpose of illustratingthe invention, there is shown in the drawings exemplary forms of theinvention that are presently preferred; however, the invention is notlimited to the specific arrangements and instrumentalities disclosed inthe following appended drawings:

FIG. 1 is a simplified diagrammatic view of one example of the scaninduction heating apparatus of the present invention;

FIG. 2 is a simplified schematic of one example of a power supply andload circuit used with the scan induction heating apparatus of thepresent invention;

FIG. 3( a) and FIG. 3( b) illustrate the application of pulse widthmodulation to change the inverter's output from full power to halfpower;

FIG. 4( a) illustrates the change in load current magnitude with achange in the frequency output of an inverter with no pulse widthmodulation;

FIG. 4( b) illustrates the change in load power magnitude with a changein the frequency output of an inverter with no pulse width modulation;

FIG. 4( c) illustrates the change in load resistance with a change inthe frequency output of an inverter with no pulse width modulation;

FIG. 4( d) illustrates the change in the Q factor of the load circuitwith a change in the frequency output of an inverter with no pulse widthmodulation;

FIG. 5( a) illustrates the relationship between an inverter's outputvoltage and load current with an inverter output frequency of 3,000Hertz and no pulse width modulation;

FIG. 5( b) illustrates the relationship between an inverter's outputvoltage and load current with an inverter output frequency of 10,000Hertz and no pulse width modulation;

FIG. 5( c) illustrates the relationship between an inverter's outputvoltage and load current with an inverter output frequency of 30,000Hertz and no pulse width modulation;

FIG. 6 illustrates the relationship between an inverter's output voltageand load current for an inverter using pulse width modulation in oneexample of the present invention;

FIG. 7 is a simplified flow chart illustrating one example of theinduction power control scheme of the present invention for controllingscan induction power as the output frequency of the inverter is changedduring the scan;

FIG. 8 is a partial simplified schematic of another example of a powersupply and load circuit used with the induction heating apparatus of thepresent invention wherein an impedance matching device is used betweenthe output of the inverter and the load circuit;

FIG. 9 is a simplified schematic of another example of a power supplyand load circuit used with the induction heating apparatus of thepresent invention; and

FIG. 10 is a simplified schematic of another example of a power supplyand load circuit used with the induction heating apparatus of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

There is shown in the figures one example of the scan induction heatingapparatus of the present invention. In FIG. 1, inverter 10 suppliessingle phase ac power to scan inductor (coil) 12 via suitable electricalconductors such as bus bars. DC input to the inverter may be from anysuitable dc power source. The inductor may comprise any type of inductorknown in the art, and may be, for example, a single or multiple turninductor, or an assembly of individual inductors that are connected toone or more ac sources of power. Workpiece 14 is held in place by ameans for moving the workpiece through the inductor, which can be, forexample, a screw drive assembly 16, with extended arms, 16 a, to holdthe ends of the workpiece. Alternatively the workpiece may be stationaryand the inductor can be moved along the workpiece, or combined andcoordinated movement of both the workpiece and inductor can be used. Ameans for rotating the workpiece, such as electric motor 18, may also beprovided for rotating the workpiece as it moves through the inductor. Aposition sensing means, such as servomechanism 20, provides positionoutput signal 21 to processor 22. The position output signal indicatesthe Y-axis position of the cross section of the workpiece that is withinthe inductor (i.e. the section of the workpiece that is effectivelycoupled with the magnetic field generated by the flow of current in theinductor).

In some examples of the invention an impedance matching device 40 may beprovided between the output of inverter 10 and the load circuit asillustrated in FIG. 8. DC input to the inverter may be as shown in FIG.2 or any other suitable method. Active and/or passive circuit componentsmay be used for the impedance matching device. By way of example, andnot limitation, a fixed ratio transformer or autotransformer, or atransformer or autotransformer with multiple taps and a tap changingapparatus to provide additional flexibility in impedance matchingbetween the output of the inverter and the load circuit may be used.Alternatively the impedance matching device may use active circuitcomponents, or a combination of active and passive circuit components,to achieve dynamic impedance matching as the impedance of a loadchanges. For example one or more inverter output power parameters and/orload electrical parameters may be sensed and inputted to the dynamicimpedance matching circuitry to make dynamic adjustments in impedance.Impedance matching device 40 may be used in combination with any otherexamples of the invention.

The workpiece can have one or more features, such as features 14 a, 14 band 14 c that may require different depths of current penetration ofinduction heating power for heat treatment and/or tempering as thosefeatures pass through the inductor. The regions of the workpiece betweenthese features may or may not require heat treatment. The multiplefeatures may be spaced apart as shown in FIG. 1 or located adjacent toeach other.

Processor 22 processes the output signal from the position sensing meansto determine the power level, frequency and time duration of inductionheating to be achieved at the inputted position of the workpiecerelative to the induction coil as further described below.

FIG. 2 is a simplified schematic of one example of an ac to dc powersupply used with inverter 10 that illustrates one method of supplying dcpower to the inverter. Rectifier section 30 comprises a full wave bridgerectifier 32 with ac power input on lines A, B and C supplied from asuitable source, such as utility power. Filter section 34 comprisescurrent limiting reactor L_(CLR) and dc filter capacitor C_(FIL).Inverter section 10 comprises four switching devices, S₁, S₂, S₃ and S₄,and associated anti-parallel diodes D₁, D₂, D₃ and D₄, respectively.Each switching device can be any suitable solid state device, such as aninsulated gate bipolar transistor (IGBT). The load circuit connected tothe output of inverter 10 comprises scan inductor L_(COIL) and workpiece14, which has regions, or features, that are coupled with the magneticfield generated around the inductor when the workpiece or inductor aremoved relative to each other. Resistance of the workpiece and the scaninductor (R_(COIL)) comprise the load resistance R_(LOAD).

FIG. 3( a) illustrates the typical output voltage waveform (FULLV_(OUT)) of the bridge inverter shown in FIG. 2 with no modulation ofthe voltage pulse width. Inverter switches S₁ and S₄ conduct during afirst time period, T₁, and inverter switches S₂ and S₃ conduct during anon-overlapping second time period, T₁, to produce the illustrated fulloutput voltage waveform with a frequency equal to ½ T₁. FIG. 3( b)illustrates the typical output voltage waveform (HALF V_(OUT)) of thebridge inverter with 50 percent duty cycle (α). Each of the inverterswitches continues to conduct for the same period of time, T₁, as inFIG. 3( a), but with the conduction periods for switches S₃ and S₄advanced by half a time period (i.e., the duty cycle is equal to 50percent) to produce the illustrated half of full output voltage. Withthis arrangement, the load is shorted every half period. Changing theduration of the overlapping conduction periods for switches S₃ and S₄results in different values for the duty cycle. Since power isproportional to the square of the supplied voltage, the power applied tothe inductor will also change as the duty cycle changes. In the presentinvention variable frequency control is achieved by changing the timeperiod, T₁, while the magnitude of the voltage (power) is adjusted bychanging the duty cycle.

The effects on the output characteristics of a power supply with varyingoutput frequency that does not use the pulse width modulation control ofthe present invention is illustrated with a baseline load circuit for aparticular workpiece. For an inverter having output power of 100,000Watts (P(f₀)) at 635 volts (V_(OUT)), and frequency (f₀) of 10,000Hertz, baseline load circuit characteristics are established as:

L₀=30×10⁻⁶ Henries inductance of the inverter load;

R₀=0.4 ohms resistance of the inverter load; and

Q₀=(2●π●f₀●L₀)/R₀=4.712 for the load circuit Q factor.

Baseline peak load current, I₀, can be calculated as 772.45 amperes fromequation (1):

$I_{0} = {\frac{V_{out}}{R_{o}} \cdot {\left( {1 - {\mathbb{e}}^{\frac{- R_{0}}{2{L_{0} \cdot f_{0}}}}} \right).}}$

FIG. 4( a) illustrates the decrease in inductor current, I(f),normalized to the baseline current, as the output frequency, f, of theinverter increases, which can be calculated from equation (2):

${I(f)} = {\frac{V_{OUT}}{R_{o}\sqrt{\frac{f}{f_{o}}}} \cdot {\left( {1 - {\mathbb{e}}^{\frac{- R_{o}}{2L_{0}\sqrt{f \cdot f_{o}}}}} \right).}}$

FIG. 4( b) illustrates the decrease in induction heating power, P(f),normalized to the baseline power, as the output frequency, f, of theinverter increases, which can be calculated from equation (3):

${P(f)} = {\frac{V_{OUT}^{2}}{2R_{o}\sqrt{\frac{f}{f_{o}}}} \cdot {\left( {1 - {\mathbb{e}}^{\frac{- R_{o}}{2L_{0}\sqrt{f \cdot f_{o}}}}} \right)^{2}.}}$

FIG. 4( c) illustrates the increase in load resistance, R(f), as theoutput frequency, f, of the inverter increases, which can be calculatedfrom equation (4):

${R(f)} = {R_{0} \cdot {\sqrt{\frac{f}{f_{0}}}.}}$

FIG. 4( d) illustrates the increase in the Q factor of the load circuitas the output frequency, f, of the inverter increases, which can becalculated from equation (5):

${Q(f)} = {Q_{0} \cdot {\sqrt{\frac{f}{f_{0}}}.}}$

FIG. 5( a) through FIG. 5( c) illustrate the generalized relationshipsin FIG. 4( a) through FIG. 4( d) for a specific example wherein pulsewidth modulation control of the present invention is not used. FIG. 5(c) graphically represents voltage and current outputs of an inverteroperating at rated full power and a frequency of 30,000 Hertz with nopulse width modulation control.

In FIG. 5( a) the output frequency of the inverter is lowered to 3,000Hertz and the current (and power) output is relatively high withoutpulse width modulation control. In the present invention pulse widthmodulation control of the inverter's output can be used to reduce thepower output of the inverter by using a relatively large duty cycle.

In FIG. 5( b) the output frequency of the inverter is at 10,000 Hertzand the power output is lower than the power output at 3,000 Hertzwithout pulse width modulation control, but still greater than the ratedfull power (current) of the inverter shown in FIG. 5( c). In the presentinvention pulse width modulation control of the inverter's output can beused with a lower duty cycle than that used at 3,000 Hertz to keep thepower output of the inverter at or below rated value.

In general, in the present invention, pulse width modulation control isused to change the inverter's output power at any operating frequencyfrom that which would occur without pulse width modulation control. Ingeneral duty cycle is decreased as frequency decreases to reduce theinverter's output power, and duty cycle is increased as frequencyincreases to increase the inverter's output power.

FIG. (6) further illustrates the characteristics of the load currentwith pulse width modulation control. When there is a non-zero inverteroutput voltage, the load current, I_(LOAD), can be calculated fromequation (6):

$I_{LOAD} = {\frac{V_{OUT}}{R_{LOAD}}{\left( {1 - {\mathbb{e}}^{\frac{R_{LOAD}}{L_{LOAD}} \cdot t}} \right).}}$

When there is zero inverter output voltage, the load current can becalculated from equation (7):

$I_{LOAD} = {I_{INITIAL} \cdot {\mathbb{e}}^{{- \frac{R_{LOAD}}{L_{LOAD}}} \cdot t}}$

where I_(INITIAL) is the magnitude of current when the inverter outputvoltage transitions to zero.

From FIG. 6, the shorter the duty cycle, the smaller the peak value ofthe load current (and power) before the load current drops when theoutput voltage is zero. Conversely the longer the duty cycle, the largerthe peak value of the load current (and power) before the load currentdrops when the output voltage is zero.

FIG. 7 illustrates a simplified flowchart for one non-limiting exampleof the scan induction heating process of the present invention. Theroutines identified in the flowchart can be implemented in computersoftware that can be executed with suitable hardware. Routine 100 inputsa workpiece (WP) scan coordinate (Y) that represents the position of theworkpiece within inductor 12. Routine 102 inputs values of power(P_(Y)), frequency (F_(Y)) and time (T_(Y)) for induction heating atposition Y. These values may be previously stored in a memory device,for example, as a lookup table based upon values established byexperimental testing of the workpiece with the apparatus. Alternativelyan operator of the scan induction apparatus may manually input thesevalues, or another method may be used to determine the requiredfrequency, power level, and, if used, the variable time value forinduction heat treatment of each position of the workpiece. Routine 104computes the required duty cycle (DC_(Y)) for the inverter output fromequation (8):Duty Cycle(in percent)=[P _(Y) /P(F _(Y))]×100,where P (F_(Y)) is calculated from equation (3) with an appropriatebaseline load circuit determined from the actual workpiece beinginduction heat treated.

Routine 106 controls switching of the power supply's switching devicesto achieve the desired output frequency and duty cycle. In thisnon-limiting example, routine 106 outputs gate inverter control signalsto the gating circuits for the inverter's switches to achieve therequired frequency, F_(Y), and duty cycle, DC_(Y). Routine 108determines whether actual measured output power is at set power P_(Y).Actual measured output power may be inputted by use of suitable sensingdevices. If actual measured power is not equal to the required setpower, then the duty cycle is appropriately adjusted in routine 110, androutine 108 repeats. If actual measured power is equal to the requiredset power, then routine 112 checks to see if set time T_(Y) has expired.If the set time has not expired, then routine 108 is repeated; if settime has expired, then routine 114 outputs a control signal to theworkpiece's positioning system to advance the workpiece to the nextincremental position for induction heat treatment and returns to routine100 for execution. In other examples of the invention the time forinduction heating at each position Y will be the same for all positionsof the workpiece within the inductor; for this arrangement, frequencycontrol and duty cycle control, as frequency changes, are used toinduction heat each position of the inductor as each position is steppedthrough the inductor at a constant rate of speed.

In other examples of the invention, movement and positioning of theworkpiece through the inductor may be predetermined, for example, wherean induction scan apparatus sequentially heat treats many identicalworkpieces. In these arrangements, power, frequency, time, and dutycycle settings at each position of the workpiece may be predetermined byexperimental testing with the workpiece and the induction scan apparatusof the present invention, and executed without further inputting orcomputing any or all of these values for each successive identicalworkpiece heat treated with the apparatus. Incremental or sequentialpositioning of parts or features of the workpiece in the inductor can beaccomplished as discrete stepped movement of the workpiece or inductor,or a combination of both, either as fine, minute steps that approachcontinuous movement of the workpiece or inductor, or coarser stepsvisually discernable as stepped movement. While the terms “selectedpart,” “multiple features,” and “locations” are used to describesections of the workpiece placed within the inductor for induction heattreatment with variable frequency and duty cycle, the present inventionincludes varying the frequency and/or duty cycle while the part, featureor location passes through the inductor. That is subsections of eachpart, feature or location may be heat treated with varying frequenciesand duty cycles as the subsections of the part, feature or location passthrough the inductor.

In other examples of the invention pulse width modulation control can beused to control the inverter's power output as the output frequency ofthe inverter varies at a given workpiece position, for example, toachieve heat treatment and tempering for a feature of the workpiece.Further sequential heat treatment of features comprising the workpieceis not limited to sequential heat treatment in the order that thefeatures are positioned in the workpiece. For example, referring toworkpiece 14 in FIG. 1, features 14 a, 14 b and 14 c may be positionedand heat treated sequentially in that order through inductor 12.Alternatively, for example, features 14 a, 14 c and 14 b may bepositioned and heat treated sequentially in that order through theinductor.

In other examples of the invention pulse width modulation control can beused to control the inverter's power output as the frequency of theinverter varies, as disclosed herein, to optimize induction heatingeffects in various types of workpieces for various types of inductionheat treatments, such as, but nor limited to, surface heat treatment,penetration heat treatment into variable depths of penetration of theworkpiece up to complete core heating, or material application heattreatment, for example, to achieve bonding of a coating material appliedto the surface of the workpiece by induction heating. The workpiece maybe a continuous workpiece, for example a strip, wire or tubing ofvarious dimensions, either hollow or solid, or discrete workpieces suchas solid sections, tubular sections, rectangular or square blocks or anyother shape requiring full or partial induction heating to achievechanges in the metallurgical structure or characteristics of theworkpiece, or to allow application of materials to the originalworkpiece, for example, in coating, brazing or diffusing.

For example a continuous workpiece, such as but not limited to a wirecan be continuously fed through one or more inductors that are connectedto the output of an inverter, either directly or via an impedancematching device. Suitable apparatus can be provided for feeding the wirethrough the one or more inductors, such as but not limited to, a supplyreel of wire on one side of the one or more inductors and a power driventake-up reel on the opposing side of the one or more inductors. Thecontinuous wire can be represented as a continuous progression of crosssections of the continuous workpiece that pass through the one or moreinductors while pulse width modulation control is used to control theinverter's power output as the frequency of the inverter varies toachieve the desired type of heat treatment for each of the progressivecross sections passing through the one or more inductors. Furthermore inother examples of the invention one or more parameters of the workpiece,such as, but not limited to, the cross sectional diameter of eachprogressive cross section of the wire can be dynamically sensed prior tobeing fed through the one or more inductors so that deviations from anominal cross sectional diameter can be sensed and used in adjusting thepulse width modulation control and frequency as the cross sectionaldiameter deviates from nominal to achieve a desired induction heattreatment in the progressive cross sections. By this method, forexample, uniform induction heated surface temperature can be maintainedeven while the diameter of the progressive cross sections of theworkpiece deviate from a nominal value. Sensing of the cross sectionaldiameter of the progressive cross sections may be accomplished, forexample, by a laser sighting array suitably positioned around the wire.Cross sectional diameter is representative of one parametric change ofthe workpiece that may be sensed for adjustment of the pulse widthmodulation control and frequency of the present invention. Further thespeed at which the wire is moving through the one or more inductors maybe varied to adjust the period of time that each one of the progressivecross sections is coupled with the magnetic field generated by currentflow through the one or more inductors to achieve the desired inductionheat treatment of each one or the progressive cross sections. In otherexamples of the invention the one or more induction coils may also movealong the length of the workpiece, either alone or in combination withmovement of the workpiece.

For discrete workpieces a series of discrete workpieces may be fedthrough the one or more induction coils by suitable conveying apparatuswhile heat treatment of each progressive cross section of each discreteworkpiece is achieved in a manner similar to the heat treatment ofcontinuous workpieces as described above. In some applications adiscrete workpiece may be individually fed through the one or moreinduction coils, or the workpiece may be held stationary and the one ormore coils may be moved along the length of the workpiece, or acoordinated movement of both the workpiece and one more induction coilsmay be used.

In other examples of the present invention, amplitude control, eitheralone or in combination with pulse width modulation control as describedabove, can be used to change the inverter's output power at anyoperating frequency from that which would occur without amplitudecontrol either alone, or in combination with pulse width modulationcontrol.

One method of achieving amplitude control is illustrated in thesimplified schematic diagram shown in FIG. 9. Rectifier 32 a may becomposed of active switching elements 33 a-33 f, such as siliconcontrolled rectifiers, so that the amplitude of the dc output voltage ofthe rectifier (input to inverter 10) can be changed by control of theactive switching elements to provide inverter output amplitude controlin combination with variable frequency output from inverter 10.

Alternatively a chopper regulator, represented by the non-limitingexample chopper regulator circuit 42 in FIG. 10 may be used to provideregulated dc power to the input of inverter 10 to change the inverter'soutput power at any operating frequency from that which would occurwithout amplitude control.

Alternatively amplitude control can replace pulse width modulationcontrol in any of the above examples of the invention.

It is noted that the foregoing examples have been provided merely forthe purpose of explanation and are in no way to be construed as limitingof the present invention. While the invention has been described withreference to various embodiments, it is understood that the words whichhave been used herein are words of description and illustration, ratherthan words of limitations. Further, although the invention has beendescribed herein with reference to particular means, materials andembodiments, the invention is not intended to be limited to theparticulars disclosed herein; rather, the invention extends to allfunctionally equivalent structures, methods and uses, such as are withinthe scope of the appended claims. Those skilled in the art, having thebenefit of the teachings of this specification, may effect numerousmodifications thereto and changes may be made without departing from thescope and spirit of the invention in its aspects.

The invention claimed is:
 1. A method of induction heating one or moremultiple features of a workpiece comprising the steps of: supplyingelectric power to at least one inductor to generate an ac magnetic fieldaround the at least one inductor; sequentially positioning each one ofthe one or more multiple features of the workpiece in the vicinity ofthe magnetic field to magnetically couple the positioned feature to themagnetic field whereby the positioned feature is inductively heattreated; matching the impedance of the supply of electric power to theimpedance of the workpiece; selectively varying the frequency of theelectric power while each one of the one or more multiple features ofthe workpiece are sequentially positioned in the vicinity of themagnetic field; and selectively varying the magnitude of the electricpower by changing the duty cycle of the electric power while each one ofthe one or more multiple features of the workpiece are sequentiallypositioned in the vicinity of the magnetic field and the frequency ofthe electric power is adjusted.
 2. Apparatus for induction heattreatment of a continuous or discrete workpiece, the apparatuscomprising: a power source having an ac output with pulse widthmodulation control; an inductor connected to the ac output to generatean ac magnetic field; a means for moving the continuous or discreteworkpiece through the inductor to magnetically couple progressive crosssections of the continuous or discrete workpiece with the magneticfield; a means for selectively adjusting the frequency of the ac outputwhen each one of the progressive cross sections is coupled with themagnetic field for induction heat treatment responsive to a parametricchange in each one of the progressive cross sections; and a means forselectively adjusting the power of the ac output by changing the dutycycle of the ac output when each one of the progressive cross sectionsis coupled with the magnetic field for induction heat treatment and thefrequency of the ac output is adjusted responsive to a parametric changein each one of the progressive cross sections.
 3. The apparatus of claim2 further comprising a means for selectively adjusting the period oftime that each one of the progressive cross sections is coupled with themagnetic field for induction heat treatment.
 4. The apparatus of claim 2wherein the parametric change comprises a change in the cross sectionaldiameter of the continuous or discrete workpiece.
 5. The apparatus ofclaim 2 further comprising an impedance matching device connectedbetween the ac output of the power source and the inductor.
 6. A methodof induction heat treatment of a continuous or discrete workpiece,comprising the steps of: supplying electric power to at least oneinductor to generate an ac magnetic field around the at least oneinductor; moving the workpiece through the inductor to magneticallycouple with progressive cross sections of the workpiece and subject theprogressive cross sections to a heat treatment; selectively varying thefrequency of the electric power while each one of the progressive crosssections are sequentially positioned in the vicinity of the magneticfield; and selectively varying the magnitude of the electric power bychanging the duty cycle of the electric power while each one of the oneprogressive cross sections of the workpiece are sequentially positionedin the vicinity of the magnetic field and the frequency of the electricpower is adjusted.
 7. The method of claim 6 further comprising the stepof selectively varying the heat treatment time that each one of the oneor more progressive cross sections is inductively coupled with themagnetic field.
 8. The method of claim 6 wherein the heat treatment issurface heat treatment, cross sectional penetration heat treatment orbonding heat treatment.
 9. The method of claim 6 further comprising thestep of matching the impedance of the supply of electric power to theimpedance of the workpiece.
 10. Apparatus for induction heat treatmentof a continuous or discrete workpiece, the apparatus comprising: aninverter having a dc input and an ac output; an inductor connected tothe ac output to generate an ac magnetic field; a means for moving thecontinuous or discrete workpiece through the inductor to magneticallycouple progressive cross sections of the continuous or discreteworkpiece with the magnetic field; a means for selectively adjusting thefrequency of the ac output when each one of the progressive crosssections is coupled with the magnetic field for induction heat treatmentresponsive to a parametric change in each one of the progressive crosssections; and a means for selectively adjusting the power of the acoutput by changing the amplitude to the dc input of the inverter wheneach one of the progressive cross sections is coupled with the magneticfield for induction heat treatment and the frequency of the ac output isadjusted responsive to a parametric change in each one of theprogressive cross sections.